Research this past year focused on the molecular characterization of pre- and postsynaptic mechanisms of neurotransmission and their relationships to the auditory system. Our analysis of postsynaptic mechanisms involved a characterization of glutamate receptor organization, dendritic targeting, and synaptic expression. To study the distribution and targeting of glutamate receptors in neurons, we used two model systems, the Purkinje neuron of the cerebellum and the fusiform cell of the dorsal cochlear nucleus. These neurons express multiple glutamate receptors and have two different excitatory inputs. We showed previously that one type of glutamate receptor, the delta receptor, is highly expressed at parallel fiber synapses, but not climbing fiber synapses in the cerebellum, and that this expression pattern is developmentally regulated. This past year we investigated the molecular mechanisms underlying this selective synaptic expression. Using yeast two-hybrid screening, we showed that PSD93 interacts with the C-terminus of delta 2. However, we also find that PSD93 is present at both synaptic populations indicating that the presence of PSD93 alone is not responsible for the selective expression of delta 2 at parallel fiber synapses. The fusiform cell of the dorsal cochlear nucleus receives different excitatory synaptic inputs on its apical and basal dendrites. We have shown that different glutamate receptors are expressed at these two synaptic populations. To investigate how this differential distribution is achieved, we studied the distribution of intracellular receptors which are presumably being transported to synapses. Our results show that the distribution of intracellular receptors matches that of the synaptic receptors indicating that dendritic transport plays a role in determining the synaptic receptor distribution. It has been proposed that developing synapses have NMDA (N-methyl-D-aspartate) receptors before they have functional AMPA (a-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid)receptors. AMPA receptors may either not be present at immature synapses or they may be functionally blocked. To address this directly, we quantified AMPA and NMDA receptors at synapses in the developing rat hippocampus using colloidal gold immunocytochemistry. Our results show that AMPA receptors are expressed at much lower levels at developing synapses than at adult synapses. The early expression of NMDA receptors raises the interesting question of how these receptors are organized in the immature synapse. At adult synapses NMDA receptors are believed to be anchored by a family of proteins through their PDZ domains. We investigated the pattern of expression of several of these proteins during postnatal development and found that SAP102 is the major protein of this family at the developing synapse, being expressed before the related proteins, PSD95 and PSD93. These results suggest that SAP102 plays a key role in recruiting NMDA receptors to the developing synapse and/or anchoring the receptor at the synapse. Because of the different structural and functional properties of the presynaptic region of hair cells, we have proposed that there are unique proteins involved in neurotransmitter release from hair cells in the organ of Corti. To identify such proteins, we have carried out a yeast two-hybrid screen of the organ of Corti using SNARE proteins as bait. We identified several proteins which interact with SNAP25 and syntaxin 1, some of which appear to be expressed predominantly in the inner ear. - synapse, glutamate receptor, protein targeting, hair cell, neurotransmitter release, immunocytochemistry